GB2358211A - Method of forming piles using a continuous flight auger and a liner - Google Patents

Abstract

A pile is constructed by boring into the soil with a continuous flight auger (14) having a hollow central shaft (15) and inserting into the soil a liner (25) which is held against rotation as the auger (14) is rotated. The liner (25) is coupled at its top end to the auger (14) for lowering through the soil and uncoupled therefrom underground, thus allowing the auger (14) to be removed whilst pumping concrete through the shaft (14) to form the pile. The liner (25) can consist of a liner extension and a main liner which are separated from eachother during removal of the auger (14). The liner (25) and auger (14) can be coupled using a device (20) including a ring bearing (31) mounted on the top of the liner (25) and a means of locking the coupling device (20) to the auger (14), which can be in the form of a pin (35) and corresponding holes.

Description

2358211 Forming Piles The present invention relates to forming piles using

coMinuous flight auger (CFA) methods, and more specifically to forming piles using permanent liners.

A standard technique for forming a CFA pile is to screw the auger to founding depth without forming an empty bore. The auger is fitted with a blade or flight which spirals helically around a hollow shaft, for tile full length of the auger. The hollow shaft is used to pump concrete to tile base of the auger, immediately filling the pile section while the auger is being withdrawn.

A positive pressure and an excess concrete flow rate are maintained, to ensure that the pile section is completely filled with concrete. After tile auger has been fully withdrawn, reinforcing elements can be inserted into tile concrete before it sets if required. (The terin 'concrete' is used here to describe concrete, mortar, mid other cementitious mixtures containing also aggregate, water, and possibly admixtures.) The flights on the auger start at or just above its boring head.

Conventionally the diameter of the boring head is slightly greater than tile flight auger above it. The base of the boring head is fitted with one or more cutting edges, teeth or picks, which cut into the soil as the auger rotates.

While tile auger is being screwed into the soil, soine of (lie soil displaced (by the self volume of the auger) is carried up the flight of the auger to (lie surface of the ground, where it is removed. The spoil is carried up the auger flight by a combination of physical phenomena - the pressure of additional spoil which is created as the auger screws deeper into the soil, (lie rotation ol' 2 the flight of the auger and the interaction with the undisturbed soil around the! side of the pile. The auger flight will normally be relatively fully loaded with: spoil, with only the excess being released at ground level.

The resistance which the soil presents to the auger will of course depend, on the nature of the soil. WIlen the soil exhibits very low resistance, the angel will screw into the ground at a rate corresponding to the pitch of its flight.

However, particularly ill competent cohesive soils, it is known (fiat if the auger is allowed to descend at its pitch rate, the spoil in the flight call becoi e densely.

packed. In this circumstance the spoil does not travel up the llight, and thus, prevents the auger screwing further into the soil. The rate of advance of a CFA is therefore normally controlled to be less than the full pitch rate.

As already mentioned, the resistance which the soil presents to the atiger depends on the nature of the soil. In many circumstances the tipper (i.e. near, ground level) levels of soil will be of a weaker nature than tile underlying soils In these instances when the boring head reaches a inore compact strata, the rate of advance will be less than that applying at the upper layers.

There are circumstances in which it may be desirable to install a leiigtli; of permanent liner for the construction of CIFA piles, over part or all of the pile length. In most cases the pennanent liner will be located in the upper part of, the pile. The circumstances requiring a permanent liner fall into two general', categories, ground conditions and local underground structures.

Considering ground conditions first, in sorne situations the soil may be:, poorly consolidated. This can occur naturally, such as in Mexico Ci(y, where the underlying volcanic ash is still consolidating. Alternatively this condition:

may be man-made, for example at a site which has been back-filled withow' adequate consolidation.

3 In both instances, after installation of any piles, ally subsequent consolidation of the soil will cause a down-drag on the piles, which increases the load oil the piles, and this load can continue to increase over a period of' 5 years. It is therefore often desirable to 'de-bond' the affected part of tile plie. This can be achieved by installing a suitable permarient liner. Colivelitiolially a steel liner coated externally with a coating such as a 6 111111 bitutneli layer is used; alternatively, a liner of smooth stainless steel, or even of plastics material, can be used. In other circumstances a twin-wall liner call be used, with the annular gap remaining empty or being filled e.g. with a bentonite suspension.

In other situations, the nature of the soil may present a risk Of COITOS1011 to the concrete of the pile. The causes of the soil contamination may be natural or man-made. For example it is well known that ill the Middle East the evaporation of water in the soil concentrates soluble salts (such as chlorides and sulphates) near the ground level. While the concrete call be designed to cater for the contamination at depth, it is extremely difficult to design it to cater for the much more aggressive conditions nearer the surface. Man-made contamination often occurs at sites of industrial waste disposal, old factories, gas works and the like.

Although special cement types (e.g. sulphate resistaiit) can sometimes be used to combat this risk, this may not always be convenient, and the corrosiori risk may be too high even for such special cements. Ill order to protect tile pile concrete from the source of corrosion, a permanent liner may therefore be used over all or part of the pile shaft.

Turning to local underground structures, these may be a basement of more usually a tunnel close to the proposed piling works. Generally the load from a pile is supported by a combination of shaft frictioti and end beamig.

4 Depending on the geometry of the pile layout and the existing underground structure, the soil can transmit some of the pile shaft friction load to tile underground structure, which is undesirable. It is therefore often considered desirable to de-bond the pile from the soil to avoid such load transmission. To achieve this, that part of the pile shaft which lies above the underground structure) is fitted with a suitable permanent liner which de- bonds that section of pile from the soil.

The conventional technique for installing liners is to first bore all open hole (either dry or flooded with drilling fluid) and then at a later stage to install a permanent liner. For the construction of bored piles, it is standard practice to install a length of temporary casing, to provide a safe working environment and also to re-locate the boring tool after each successive boring increment. Thus, when a permanent liner is introduced, there is an annular gap between it and the temporary casing. It is a normal requirement that this annular gap is filled with, e.g. a grout. In contrast, CFA (Continuous Flight Auger) piles are. conventionally constructed without the use of any temporary casing. It Call be difficult or impractical to install a permanent liner by ineans of CFA equipment, alone, although techniques mentioned above are available.

The general objective of the present invention is to provide an improved: technique of inserting such liners.

According to the invention there is provided a method of constructing i, pile by boring into the soil with an auger having a continuous flight and a. hollow central shaft and inserting into the soil a liner which is field agai'iist, rotation as the auger is rotated, and then removing the auger while pumpingi concrete through its shaft, wherein the liner is coupled at its top end to the auger for lowering through the soil and then uncoupled therefroni.

In preferred forms of the invention, the CFA rig includes a coupling device consisting of an upper unit and a lower unit connected together by a ring bearing. The upper unit can be coupled to the auger, so that as the auger moves downwards as it drills into the soil, it carries the coupling device dowil with it. The lower unit is coupled to the liner, so that as (lie coupling device is carried downwards, the liner is forced into the soil. Theupperunitnecessarlly rotates with the auger; the lower unit is coupled to the finer, and is restrained froin rotation, so preventing the liner froin rotating.

The CFA rig is used to install the liner at the beginning of the construction of the CFA pile. The top of the liner l s attached to the underside of the lower unit of the coupling device. The upper unit is connected to the flight auger, causing this section to rotate with the auger. The lower uint therefore presses the liner into the ground while the auger is screwed into the ground.

After the liner has been installed to a predetermined depth, the upper unit of the coupling device is released ftoin the auger, and the auger is thell screwed to the full desired depth in the normal manner, excavating the pile to the full required depth. During this further excavation, the finer reniains in place; there is no downwards force on it during this stage, and it is held against rotation by the lower unit of the coupling device, to which it reinailns attached. The auger moves downwards through the coupling device; the lower unit of the device may be locked to the upper unit during this stage if desired to prevent its rotation.

During the concreting phase, the auger is withdrawn as normal, while pumping concrete through its shaft. When the auger has been withdrawn to a height where it was detached from the coupling device, that device is re6 attached to the auger. The concreting phase is then re-coniinenced, until complete removal of the auger.

The liner is detached from the lower unit of the coupling device, conveniently either when the coupling device is released froin the auger or when it is being re-attaclied. The liner therefore remains in the ground as the withdrawal of the auger is completed.

In some circumstances, it may be desirable to install a permanent liner with its top end below ground level. This can be achieved by using a liner assembly consisting of a main liner with a detachable upper extension. 'File liner assembly is treated as a temporary easing during the drilling phase, witil the liner extension being attached to the coupling device and remaining attached to it when the coupling device is released from the auger. During the concreting phase, when the coupling device is re-attached to the angel., (lie linel. extension is detached from the main liner, so that the main liner remains in tile ground while the liner extension is carried up and withdrawn by (lie auger, The liner extension may be detached manually fl-om the main liner if tile linkage between the two parts is accessible. Alternatively, the two pails may be linked by a bayonet-type connection or the like which will automatically release when the liner extension is pulled upwards way from the inain liner.

The invention thus provides a method of constructing a CFA pile with a permanent liner, either at or below ground level, in the site and soil conditions described above.

The invention also provides pile forming apparatus coinprising aii aiigci:, and a coupling device including a ring bearing and mounted at the top of the:

1 1 7 liner for coupling a pile liner to the auger, and means for attaclung the coupling device to the auger.

Pile fanning machinery in accordance with the invention and for performing the method of the invention and its method of operation will now be described, by way of example, with reference to the drawings, in which:

Figs.]A and 1 B together are a general view of the iiiaciiiiiei-y; Fig. 2 is a more detailed view of the auger and associated coniponellts; and Fig. 3 is a detailed view of a part of the auger flight.

Figs. IA and IB show pile forming machinery 10 consisting of a crawler unit 11 carrying a mast or column 12. The mast 12 has mounted on it an auger driving unit 13 which carries an auger 14. The driving unit 13 includes a motor for rotating the auger 14, and can also move itself up and down (lie inast. The auger passes through a guide collar 18 mounted on the bottoin end of the mast 12. The auger 14 consists of a hollow central shaft 15 around whIch 11 continuous blade or flight 16 is attached, and may be forined froin a nuinber of' sections bolted together. The mast 12 carries concrete supply tubing 17 at its top end, coupled to the top end of the auger shaft by flexible tubing (not shown). During construction of the pile, the unit 13 rotates the auger and is lowered as the auger is screwed into the ground. When the auger has reached the desired depth, the unit 13 pulls the auger up out of the hole (with or without rotation). While the auger is being withdrawn froin the pile, a punip (riot shown) pumps concrete down the hollow shaft 15 to fill the section vacated by the auger, thus forming the pile.

A liner 25 is mounted round the auger 14 and coupled to the auger 14 by a coupling device 20 which operates to couple the liner to the auger, so that the liner is forced into the ground as the auger descends, while holding the liner 8 against rotation. The coupling device comprises a mechanism for transmitting downward force while preventing rotation.

The coupling device 20 is shown in more detail in Fig. 2 and consists generally of an upper or rotating unit 21 and a lower oi- non-rotating unit 22 coupled together by a ring bearing 3 1.

Considering first the lower or non-rotating unit 22, this unit comprises a sleeve 28 with a flange 32 and reaction at-ins or plates 27 which can slide fi-eely up and down the mast 12 but restrain the lower unit from rotating. Tfus unit -otational forces has to be coupled to the liner so as to hold the liner against i from the auger, and, the case of a liner extension, to pull the liner extension out of the ground as the auger is extracted ftom the ground.

The liner 25 may be retained in sleeve 28 with locking pins 29. To provide better coupling, however, locking keys may be used, each locking, key consisting of a rectangular key element which fits into aligned rectangular holes in the lower unit 22 and the top of the liner and can be bolted or screwed into, position. Alternatively or additionally, the lower unit 22 may be provided with splines or castellations which interlock with corresponding splines olcastellations on the liner or liner extension, to provide better anti- rotational coupling.

Turning now to the upper or rotating unit 2 1, this comprises a sleeve 334; with the auger 14 passing axially through it. A flange 33 is attached to the: sleeve 34, and the bearing 31 is attached between flanges 32 and 33; if desired, an outer shroud may be attached to the outer edge of flange 333 to help protect., the ring bearing. The sleeve 34 extends down inside sleeve 20 of the loweiassembly, as shown, to help prevent ingress of soil into the bearing 3' 1.

9 The ring bearing 31 must retain the two units together. If the device is to be used solely for installing permanent liners at ground level, the longitudinal forces on the ring bearing may not be large, but If it is to be used for installing liners below ground level, then considerable force illay be required to pull the liner extension out of the ground. A series of clips or latches 30 may be provided around the coupling device as shown to trallsmit such forces between the upper and lower units of the deviceThe flanges 32 and 33 have holes through which a pin 35 can be inserted to lock the rotating and non-rotating units of the coupling device together if' desired. The detail of this locking assembly is not critical to the function of the equipment. Either or both of the flanges may have a series of holes around them so that the two units can be locked together with only minor adjustment of their relative angular positions.

The sleeve 34 of the upper unit 21 has two mounting apertures 36 in it as shown, diametrically opposite each other and vertically offset by lialf (lie pitch of the auger flight 16. The auger flight 16 has attached to or formed in it a pail, of sockets 37; as shown in Fig. 3, the sockets are preferably located generally beneath the upper surface of the flight. Additional sets of sockets may be fitted at other levels of the auger flight. A pair of bars 338 can be inserted through the mounting apertures 36 of the upper unit 2 1 and into the sockets 37 on the auger, so locking the upper unit 21 to the auger. A pair of pins 39 call be inserted through the sockets 36 and bars 38 to hold the bars in place.

To form a pile in soil using the present techniques, the pile forining machinery is assembled as shown, with a liner 25 around the auger 14. TO ensure that the liner will be drawn down by the auger, the position of the lincr 25 is adjusted so that the boring head 40 of the auger 14 protrudes a short distance beyond the bottom end of the liner. The length of the liner is chosen to match the required depth down the pile, which will be the depth to which debonding is required.

Before drilling starts, the upper unit 21 of the coupling device 20 is locked to the auger 14. When drilling is started and proceeds, this upper, lilt will rotate with the auger, and be carried down as the auger penetrates tile soil. The liner 25 is held against rotation, but is forced down the hole by couplitig E device, which presses on the upper end of the liner via the upper unit 2 1, the ring bearing 3 1, and the non-rotating unit 22. The permanent finer 25 therefore lines the hole as the pile is constructed.

When the liner has been installed to the desired depth, the drilling is interrupted and rotation of the auger is stopped. The upper rotating ullit 2 1 is now disengaged from the auger, by withdrawing the bars 3)8, and locked to the lower unit 22 by inserting the pins 39. Drilling is theil restarted. The 1111U25 is, as before, held against rotation by the reaction arms or plates 27. Since the upper rotating unit 21 has been disengaged from the auger, the auger can move freely through it. The auger therefore continues to rotate and move downwards as it continues to penetrate the soil, while [lie permanent Illier and its attached upper assemblies remain in position with no further rotation oi" downward movement.

After the auger has reached the required depth, concrete is pumped through shaft 15 of the auger, which is withdrawn at a controlled rate such thati, no void is created. For the initial pail of this process, the coupling device 'M reinains in position. When the sockets 37 of the a.kigei- rise to the level of the: apertures 36 in the upper unit 21, however, the concretitig process is: interrupted. Withdrawal of the auger is halted, and the tipper tinit 21 is: coupled again to the auger by means of the bars 38. (A slight rotation or 11 vertical adjustment of the auger may be required, to align the sockets 37 witli the mounting apertures 36).

If the liner is to be a permanent liner at ground level, at tins ponit it is detached froin the underside on the lower unit of the coupiflig device by withdrawing pins 29. If the liner is to be a permanent finer below ground level, it will be part of a liner assembly with a liner extension which will left attached to the lower unit.

Withdrawal of the auger and injection of concrete is tlien restarted.

This results in the withdrawal of the coupling device, which is carried up by the auger by means of its upper unit. As the auger and coupling device are withdrawn, so concrete is injected into the pile under positive pressure, until the auger has been completely removed. If a liner extension is being withdrawn, it will be disconnected, either manually or automatically, froill the main liner, which will remain in the pile bore below ground level.

Once the auger has been fully withdrawn, the pin 35 is withdrawn (if it was not withdrawn earlier), and the auger is rotated again in the liner to clear spoil from the region of (lie auger inside the liner. The auger can of course be rotated in either direction for this. However rotation in the opposite direction (to that used for screwing the auger into the ground) may be niore effective in clearing the spoil, but will require the auger drive unit 13 to have bi-directiotial drive.

12

Claims (1)

1. Claims

   1 A method of constructing a pile by boring into the soil with an auger having a continuous flight and a hollow central shaft and inserting into (lie soil a liner which is held against rotation as the auger is rotated, and then removing the auger while pumping concrete through its shaft, wherein the liner is coupled at its top end to the auger for lowering through the soil and then uncoupled therefrom.

   2 A method according to claim 1 wherein the liner consists of a liner assembly comprising a liner extension and a main liner which are separated from each other during removal of the auger.

   3 A method of making a pile substantially as herein described witli reference to the drawings.

   4 Pile forming apparatus comprising an auger and a coupling device including a ring bearing and mounted at the top of the finer for coupling a plie liner to the auger, and means for attaching the coupling device to tile auger.

   Apparatus according to claitn 4 including a locking device for locking:

   the coupling device to the liner.

   6 Apparatus according to claim 5 wherein the locking devices comprise v pin and holes in the coupling device and the top end of the lincr.

   7 Apparatus according to any of claims 4 to 6 wherein the attachment, means comprise mounting rneans formed oil the coupling device, socket means, I i 13 mounted on the auger flight, and bar means insertable through the mounting means into the socket means.

   8 Apparatus according to claim 7 wherein the attachment ineans also 5 include pin means for holding the bar means in the socket iricans.

   9 Apparatus according to any of claims 4 to 8 and including a linel. assembly comprising a liner extension and a main liner coupled togetlier and uncouplable below ground level. 10 10 Pile forming apparatus substantially as herein described with reference to the drawings.

   11 Any novel and inventive feature or combination of features specifically 15 disclosed herein within the meaning of Article 41-1 of the International Convention (Paris Convention).

   Amendments to the claims have been filed as follows Claims A method of constructing a pile by boring the soil with an au(,-ei-.

   I I I 1 0.

   having a continuous flight and a hollow central shaft and inserting into the solil:

   a liner which is held against rotation as the auger is rotated. and then removing ZI the auger while pumping concrete through its shaft, wherein the liner is coupled at its top end to the auger for lowering through the soil and then uncoupled therefrom.

   2 A method according to claim I wherein the liner consists of a liner assembly eomprising a liner extension and a main liner which are separated from each other during removal of the auger.

   with A method of making a pile substantially as herein descn reference to the drawings.

   4 Pile forming apparatus comprising an auger and a coupling device including a ring bearing and mountable at the top of a pile liner for coupling the liner to the auger, and means for attaching the coupling device to the auger.

   Apparatus according to claim 4 including locking means for locking the coupling device to the liner.

   6 Apparatus according to claim 5 wherein the locking ineans comprise a! pin and holes in the coupling device and the top end of the liner.

   7 Apparatus according to any of claims 4 to 6 wherein the attachment:

   )0 means comprise mounting means formed on the coupling device, socket means.

   mounted on the auger flight, and bar means insertable through the mounting means into the socket means.

   8 Apparatus according to claim 7 wherein the attachment means also 5 include pin means for holding the bar means in the socket means.

   9 Apparatus according to any of claims 4 to 8 and including a liner assembly comprising a liner extension and a main liner coupled together and C1 uncouplable below ground level.

   to Pile forming apparatus substantially as herein described with reference to the drawings.